1. Field of the Invention
This invention relates generally to magnetic recording disk drives, and more particularly to a disk drive that includes a head-positioning servo control system that has compensation for generally high-frequency mechanical and electrical disturbances.
2. Description of the Related Art
Magnetic recording hard disk drives use an actuator, typically a rotary voice-coil-motor (VCM) actuator, for positioning the read/write heads on the data tracks of the recording disks. The disk drive has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track and move them to the desired track for reading and writing of data.
The servo control system is designed for optimal response to generally low-frequency mechanical disturbances. However, disk drives may experience high-frequency mechanical and electrical disturbances to which the servo control system cannot adequately respond. Also, the servo control system may detect such a high-frequency mechanical disturbance (MD) or electrical disturbance (ED) and respond as if it were a low-frequency disturbance, a problem referred to as “aliasing”.
A typical MD that occurs at a frequency outside the design range of the servo control system is rotational vibration (RV). RV may arise internally, such as from motion of the VCM actuator, or externally, such as from shocks to the frame supporting the disk drive or from the movement of other disk drives when the drives are mounted together in a disk array system. RV compensation is a method that uses sensors (typically accelerometers) to detect RV and improve the PES by canceling the off-track motion induced by the RV. The RV sensor signal is input to a RV feedforward controller that creates a RV feedforward compensation signal that is summed with the control signal to the VCM actuator. The use of a RV sensor and feedforward compensation is described by Jinzenji et al., “Acceleration Feedforward Control Against Rotational Disturbance in Hard Disk Drives,” IEEE Transactions on Magnetics, Vol. 37, No. 2, March 2001, pp. 888-893; and M. T. White et al., “Increased Disturbance Rejection in Magnetic Disk Drives by Acceleration Feedforward Control,” Proceedings of the 13th Triennial IFAC World Congress, Jun. 30-Jul. 5, 1996, San Francisco, Calif., pp. 489-494.
However, the RV sensor output may include noise not related to RV. The source of noise may be electrical or sensor-related, such as non-rotational vibrations detected by the sensor's cross-axis sensitivity, and/or spurious signals generated as a result of physical distortion of the sensor itself. Thus if the RV compensation is enabled when the disk drive is not being subjected to RV disturbances, the servo control system performance may be degraded.
A typical ED that occurs at a frequency outside the design range of the servo control system is noise in the power supply voltage applied to the VCM driver that sends control current to the VCM actuator. This noise, typically a ripple voltage of the nominal power supply voltage, will cause the VCM driver to generate a control current with noise, resulting in undesirable mechanical movement of the VCM actuator. Pending application Ser. No. 12/036,478, filed Feb. 25, 2008 and assigned to the same assignee of this application, describes a disk drive with ED feedforward compensation for an ED to the VCM driver from power supply voltage noise. However, if this ED compensation is enabled when the disk drive is not subjected to noise from the power supply voltage, or the signal is an aliased version of an existing high-frequency noise due to the lack of a pre-designed or correct anti-aliasing filter, the servo control system performance may be degraded.
An additional problem has been discovered if both an MD in the form of RV, and an ED in the form of power supply voltage noise, are present simultaneously. Specifically, the movement of the VCM actuator and/or direct coupling into the RV sensor system induced by power supply voltage noise can cause the RV sensor to erroneously sense RV. This causes the RV feedforward compensation to inject noise into the servo control system.
What is needed is a disk drive that has compensation for both mechanical and electrical high-frequency disturbances outside the design range of the servo control system, but wherein either or both of the MD and ED compensations can be disabled when not needed.